1. Field of the Invention
The present invention generally relates to a multi-layered printed circuit board embedded with a filter and, more particularly, to a composite multi-layered printed circuit board formed of at least a high dielectric material stacked with at least a low dielectric material. At least one interdigital capacitor is disposed on a low dielectric material and metal electrodes of the interdigital capacitor are located on the same plane such that the area of the metal electrodes or the spacing between the metal electrodes can be adjusted in advance to precisely control the electrical properties such as the center frequency and the transmission loss of the filter.
2. Description of the Prior Art
In recent years, it has been feasible to manufacture radio frequency (RF) microwave devices such as RF filters using printed circuit boards (PCBs). However, with the fast development in modern technologies, the quality as well as the performance of the filters using PCBs is critically emphasized so as to precisely control the electrical characteristics such as center frequency, band-width, return loss, insertion loss and out-band rejection. Moreover, in order to consider the deviation caused during PCB manufacturing prior to circuit design for systematic evaluation, it is required a PCB structure, in which the electrical characteristics can be easily controlled.
In U.S. Pat. No. 6,448,873, entitled “LC Filter with Suspended Printed Inductor and Compensating Interdigital Capacitor”, a spiral inductor coupled in parallel with an interdigital capacitor is used to form a LC resonance circuit and a filter circuit can be thus formed using a plurality of the LC resonance circuits coupled serially with a capacitor. The capacitor can be a surface mounted device (SNM). Even though U.S. Pat. No. 6,448,873 discloses a filter embedded in a PCB using interdigital capacitors, the interdigital capacitors are for LC resonance, not for the serial connection.
Moreover, in U.S. Pat. No. 6,577,208, entitled “Radio Frequency Filter”, wherein a ceramic substrate is used and thus a modular filter is an SMD, the utilized interdigital capacitors are coupled serially with a wire wounded inductor. However, the application of the interdigital capacitors in the filter is not stated.
Therefore, there exists a need in providing a multi-layered printed circuit board embedded with a filter, the multi-layered printed circuit board using a composite multi-layered printed circuit board formed of at least a high dielectric material stacked with at least a low dielectric material. At least one capacitor is an interdigital capacitor disposed on a low dielectric material. Metal electrodes of the interdigital capacitor are located on the same plane such that the area of the metal electrodes or the spacing between the metal electrodes can be adjusted in advance to precisely control the electrical properties such as the center frequency and the transmission loss of the filter. Problems resulting from alignment errors caused in manufacturing the composite multi-layered printed circuit board can also be prevented.